The Best AI Japanese Translation Tools—Tested and Ranked!

Where Machine Translation lacks is often in its ability to mimic natural language. This makes Machine Translation a better choice for technical writing versus anything creative. 

Generative AI has been a huge step forward in automated translation, with some tools reaching close to human-level accuracy. These tools are trained on massive amounts of data, allowing them to learn the nuances and subtleties of different meanings through context. They can also embellish or change your translations with prompts, whereas direct Machine Translation will never give you back more than it receives. 

Tools that work more like Machine Translation would be Google Translate or DeepL, whereas Generative AI tools are more like ChatGPT or Gemini. 

Testing AI Japanese translation tools

So which AI translation tool works the best when translating from English into Japanese? We tested 5 different, popular tools to find out! 

The 5 tools we decided to test were:

• Google Translate
• DeepL
• Claude 3.5
• ChatGPT-4o
• Gemini

We chose these 5 tools based on their:

• Popularity
• Accessibility
• Available customizations
• Reputation

Each tool is free (or has a free version) to maintain maximum accessibility, and has a range of customization options available. This way, we can test tools with no customization available such as Google Translate, against tools with some customization available such as DeepL, and full customization such as the generative AI tools Claude 3.5, ChaptGPT and Gemini.

How We Conducted Our test

To test these tools, we chose a passage from one of our previous blogs as the ‘sample’ and let each tool translate it twice—once with additional information such as glossaries or a prompt, and once without. We then checked how the tools fared in each situation and if using a prompt or glossary was able to improve the results. 

To keep things fair between the generative AI tools that allow prompts, we used the same prompt across each tool. 

After running the sample text through each AI translation tool both times, we had a native Japanese speaker review the translations and rate them based on the following criteria:

• Is the translation grammatically correct?
• Is the translation accurate in terms of content?
• Does the translation have a natural flow?
• Is the tone of voice of the translation appropriate for the type of text? (In this case, a marketing blog)
• Does the translation come out in proper format?

Each criterion was ranked on a scale of 1 to 6 (with 6 being the highest) and then given a total score out of 30.

Let’s see how they did!

The score breakdown

Google Translate

Our first test was with Google Translate. While it’s free, easy to use, and is one of the most popular translation tools out there, it scored the lowest when translating our test passage, sometimes giving unnatural or weird expressions and not formatting the resulting text in a way that’s easy to read.

Scores without prompts or glossary:

• Is the translation grammatically correct? 3/6
• Is the translation accurate in terms of content? 3/6
• Does the translation have a natural flow? 2/6
• Is the tone of voice of the translation appropriate for the type of text? 4/6
• Does the translation come out in proper format? 3/6

Total: 15

Score with prompts or a glossary: 

• N/A

Having no customization options, there isn’t much you can do to improve the results of a Google Translate translation. So while Google Translate might be okay for getting the gist of what a piece of text says, we wouldn’t recommend using it for much more than that. 

DeepL

DeepL is another popular online translator with both a free and paid version. We tested the free version, which allows only 1500 characters to be translated at a time and has a limit on the number of documents you can translate (maximum 3 per month). 

With the free version, you also have access to the glossary feature. So for the second round, we included a glossary to see how much it would help improve the final translation.

The drawback for this approach is that if you don’t have a native speaker to create your glossary, it’s not very useful. But if you often translate the same types of text, having a glossary created once to use for all future translations may be a good option. 

Scores without prompts or a glossary:

• Is the translation grammatically correct? 4/6
• Is the translation accurate in terms of content? 3/6
• Does the translation have a natural flow? 4/6
• Is the tone of voice of the translation appropriate for the type of text? 5/6
• Does the translation come out in proper format? 3/6

Total: 19

Scores with a glossary:

• Is the translation grammatically correct? 4/6
• Is the translation accurate in terms of content? 4/6
• Does the translation have a natural flow? 3/6
• Is the tone of voice of the translation appropriate for the type of text? 5/6
• Does the translation come out in proper format? 4/6

Total: 20

Without the glossary, DeepL struggled the most with accuracy and formatting. While, like other translators, it included a couple of weird phrases, it also omitted a subtitle from the original text when ran without the glossary. While the end meaning still ended up the same, these kinds of large formatting changes or omissions can be quite severe when trying to create a piece that matches the original. 

Adding the glossary forced DeepL to include the subtitle, so we were able to improve the accuracy of the translation with this customization. However, while improving the accuracy of the translation it also ended up prompting DeepL to have a less natural flow as it tried to use the required terms.  

On the positive side for this tool, the tone of voice remained consistently appropriate across both versions, and while including a couple of strange expressions, it didn’t produce any major grammatical errors.

ChatGPT-4o

ChatGPT-4o is OpenAI’s generative AI tool that, while not specifically for translation, can be used for translation and supports 50 languages. As with all AI translators, its accuracy varies between language pairs, but it allows you to create prompts to help guide the translation output. 

And while there is a paid version, we used the free version for this test. 

Scores without prompts or a glossary:

• Is the translation grammatically correct? 3/6
• Is the translation accurate in terms of content? 4/6
• Does the translation have a natural flow? 2/6
• Is the tone of voice of the translation appropriate for the type of text? 4/6
• Does the translation come out in proper format? 5/6

Total: 18

Scores with using a prompt:

• Is the translation grammatically correct? 5/6
• Is the translation accurate in terms of content? 3/6
• Does the translation have a natural flow? 3/6
• Is the tone of voice of the translation appropriate for the type of text? 5/6
• Does the translation come out in proper format? 5/6

Total: 21

Without a prompt, ChatGPT had some trouble with natural flow, using odd expressions and word choices which affected its score. However, it still delivered fairly impressive results in terms of accuracy, tone, and formatting.

In the second attempt when we asked ChatGPT-4o to re-do the same translation with a prompt, it performed better in terms of natural flow, grammar and tone of voice, but a bit worse in terms of accuracy. This is likely because we gave the platform more freedom to deviate from the original text to fit our additional requirements. So while it wasn’t as directly accurate, the translation was able to flow better. Overall, using the prompt did improve the result!

Claude 3.5

Claude 3.5 is Anthropic’s latest generative AI tool released as a core competitor for OpenAI. It is most known for its ability to tackle complex reasoning tasks. Similarly to ChatGPT, it’s not a translation tool exclusively, but can be used to translate text in over 50 languages. 

It has both a free and a paid version, but we used the free version for our test. 

Scores without prompts or glossary:

• Is the translation grammatically correct? 5/6
• Is the translation accurate in terms of content? 5/6
• Does the translation have a natural flow? 4/6
• Is the tone of voice of the translation appropriate for the type of text? 6/6
• Does the translation come out in proper format? 6/6

Total: 26

Scores with using a prompt:

• Is the translation grammatically correct? 3/6
• Is the translation accurate in terms of content? 2/6
• Does the translation have a natural flow? 3/6
• Is the tone of voice of the translation appropriate for the type of text? 6/6
• Does the translation come out in proper format? 6/6

Total: 20

Surprisingly, Claude 3.5 did significantly better with no prompts than it did with prompts. When given no prompts, our native-speaking analyst commented that there was only one small thing they would have changed, but other than that it worked really well. 

When given prompts, things started to go a bit downhill. The tool ended up bringing in some unnatural expressions and even included a sentence that didn’t make any sense at all. This affected the score greatly for accuracy and grammar, but natural flow as well. 

But one highlight for both versions was the formatting. The resulting translation was well formatted and made sense in comparison to the original text both with and without a prompt. Claude 3.5 even included some more complex formatting elements such as bullet points, something none of the other tools used but that was present in the original English article. 

Gemini

Gemini (formerly Bard) is Google’s AI model that, similarly to the previous two, is not specifically a tool for translation but is able to handle translation tasks. Gemini’s core strength is multimodal AI reasoning, meaning it can make sense out of a mixture of data sources (text, image, video, etc.). 

There is a free and a paid version, but we used the free version for our test. 

Scores without prompts or glossary:

• Is the translation grammatically correct? 5/6
• Is the translation accurate in terms of content? 4/6
• Does the translation have a natural flow? 4/6
• Is the tone of voice of the translation appropriate for the type of text? 3/6
• Does the translation come out in proper format? 1/6

Total: 17

Scores with using a prompt:

• Is the translation grammatically correct? 4/6
• Is the translation accurate in terms of content? 3/6
• Does the translation have a natural flow? 3/6
• Is the tone of voice of the translation appropriate for the type of text? 5/6
• Does the translation come out in proper format? 5/6

Total: 20

Without a prompt, Gemini outperformed ChatGPT in terms of language, but the complete lack of formatting (the entire translation came out as a single paragraph) made the resulting translation very difficult to read. The score for grammar though, was particularly impressive, tied with Claude 3.5. 

With a prompt, we experienced similar problems with accuracy as we did with both other Generative AI models, ChatGPT and Claude 3.5. But the tone of voice and the formatting improvements were night and day. As with all of the tools, there were a couple of strange expressions included, but otherwise, we saw no major issues. 

Do I need to use prompts or glossaries when running AI translation?

Using prompts and glossaries with Japanese AI translation tools can help improve tone and formatting—if done right. However, we’ve found that focusing on these elements sometimes sacrifices translation accuracy.

It seems that prompts may encourage the AI to use more creative language or less common phrases in an attempt to match industry-specific terms or the right tone for a marketing blog. Over time these tools will likely improve, but for the moment, more creative translation requests or requests with uncommon terminology may still be difficult for them to accommodate.

Therefore, it’s important to always check the resulting translation with a native speaker before publishing any AI-translated content, but let’s dig a little deeper into what happened during our test. 

There were several words and phrases that the AI translation tools consistently struggled with when translating them into Japanese. These were:

• “often featuring big-name celebrity endorsements” 
•  “Short-form content caters to this reality” 
• “Mobile Dominance”

If using a glossary, mistakes on difficult words can be avoided as preferred translations are provided before running the translation. This approach worked well with DeepL, where the score from accuracy went from 3 to 4 when using a glossary. 

Using prompts on the other hand, helped correct the tone of voice and formatting across all generative AI platforms, but as explained above, not all scores increased with the use of prompts. 

So is using a prompt really beneficial? The answer might be yes, but it’s not a one-and-done game.

To get the most out of a generative AI tool, you may have to work with it a bit. We recommend initially giving the tool a detailed prompt, and then asking it, step by step, to fix areas with strange terms or awkward sentences. We found this might be the best way to get a usable final result. 

How to create a good prompt

A good prompt is straightforward, simple, and contains the contextual information an AI needs to understand a piece of content. 

The prompt in our test had all of the following elements:

• The target language
• Tone of voice guidance
• The type of content we were translating (blog)
• The industry that specific terms may be coming from (marketing)
• A sample blog in the target language to reference for tone of voice and style 

For inspiration on making a prompt of your own, our prompt followed the below structure:

Please translate the following (type of content, i.e. ‘blog’) content on (topic of content) into Japanese. Remember to also use (industry, i.e. ‘marketing’) industry-specific terminology. Use a (tone of voice guideline, i.e. ‘positive’, ‘professional’) tone of voice. Please also refer to the article at the following link as a reference (insert link).  “Insert text to translate here between quotations”.

Which AI translation tool works best for translating to Japanese

The top-scoring AI tool for Japanese was Claude 3.5, without using any prompts. With no additional information, the tool was able to create the most accurate translation, only scoring below a 5 or 6 on ‘natural flow’.

With this result, we recommend Claude 3.5 the most for anyone with no Japanese knowledge who is just looking for the most accurate translation. 

Drawbacks in using AI to translate into Japanese

The most obvious difficulty with using AI translators of any kind is being able to judge the results. Is the translation any good? Without having a native speaker to review it, it’s impossible to tell. 

This becomes even more difficult when giving generative AI tools the freedom to stray from the exact source and letting them try to mimic more natural expressions. Sometimes they get it, but sometimes they don’t. Particularly for less common languages such as Japanese where the accuracy of all AI tools just isn’t as good as more common language pairs such as English into Spanish. 

We recommend always having a native speaker, or even better a professional translator, check any AI translations before using them publicly. Mistranslations can cause damage to your brand, lower consumer trust, promote misinformation and even unintentionally offend audiences. So while it may be a good place to start, post-editing any AI-generated translation is a must!

Up Your Translation Game with LocaRise

Having an accurate translation isn’t usually enough when it comes to localizing content for a new market. There are also cultural barriers that exist and also need to be addressed if you’re going to create content that really resonates with the local audience.

Our survey revealed that 96.9% of brands are underperforming in Japan compared to their global results. Why? A lot of them underestimated the importance of proper localization. You don’t want to fall victim to poor localization ruining your chances in the Japanese market, and neither do we!

So, with over 15 years of experience in the Japanese market and extensive testing with 84 clients, we identified over 232 situations & scenarios that are indicative of the unique Japanese culture and may affect your brand. We then distilled those down to 8 focal points (we call these “lenses”) for you to address. 

If you need help localizing your brand for the Japanese market, get in touch! Our international team of native English and Japanese speakers are ready to make sure that nothing gets lost in translation.

Steal Our Best Ideas

Actionable insights straight from our data

Here are a couple quick discoveries we’ve pulled from the data of our latest projects. Why? To help you make the changes you need to gain traction in the Japanese market! As an agency, we are always digging deeper and searching for those little yet significant tweaks that will push our clients to the next level of success. If you need a partner to help you identify and implement changes like these on a monthly basis, let us know!